I developed consistently high performing hybrid perovskite solar cells in p-i-n and n-i-p configuration with power conversion efficiency ≈16%. High quality thin films of different compositions of hybrid perovskites were prepared by solution processing and their photovoltaic properties were studied. Charge extraction kinetics of the most widely used perovskite photoactive layer of CH3NH3PbI3 to the charge transport layers of NiO and PEDOT: PSS were investigated using time-resolved photoluminescence and transient absorption measurements. The study revealed that in the most commonly used hole extraction layer of PEDOT:PSS, at illumination intensities of 1 Sun, the charge extraction is interface limited giving a power conversion efficiency of only 7.5 % where as in NiO, a faster charge extraction, on the time scale of 300 ps is occurring with achieved PCE of ≈ 13%.
Because of the simple solution processing method used for the fabrication of the hybrid perovskites, multiple length scales of heterogeneity in properties exist in these materials. These heterogeneities can adversely impact the stability and the performance of perovskite photovoltaic devices made from these materials. It is a real challenge to identify the origin and spatial distribution of these grain-to-grain inhomogeneities, and their correlation with functional properties. Using cathodoluminescence hyperspectral imaging method I have identified the heterogeneity in light emission from the promising optoelectronic materials of CsPbBr3 and CsPbBr3:KI with a high spatial resolution (~50 nm) and related this to the composition and the local strain. Our study showed that grain boundaries are under compressive strain and are associated with higher non-radiative recombination losses. Also, in the mixed halide composition especially the iodide rich phases exist in the grain boundary regions. Our study provided the first visual evidence for the existence of nanoscale heterogeneity in composition, both laterally and vertically in CsPbBr3 and its mixed halide composition.
Charge extraction layers that can be processed at temperatures compatible with flexible substrates such as polyethylene terephthalate (PET) were developed. The metal oxide-based charge transport layer of NiO (for hole extraction), SnO2 and ZnO (electron extraction layers) were developed to obtain efficient charge extraction for both p-i-n and n-i-p device architectures. Compared to the commonly used hole extraction layer (HEL) of PEDOT:PSS, hybrid perovskite solar cells with NiO nanoparticles as HELs showed enhanced shelf-life stability by preventing the permeation of moisture into the active layer. The low temperature processability (at 100 oC) of NiO permitted the fabrication of flexible perovskite solar cells with power conversion efficiency close to 10 %. A new methodology of transport layer optimization using surface photovoltage measurement of partial heterostructures of perovskite solar cells was established. In collaboration with the University of Glasgow, BODIPY based novel hole transporting materials were developed which has the potential to be employed in both p-i-n and n-i-p configuration with appropriate surface passivation using PFN molecules. This novel cost-effective hole transporting material can replace the expensive and complexly prepared Spiro-MeOTAD but without compromising on power conversion efficiency (≈ 15 %). In collaboration with Chemists from the School of Chemistry, University of St Andrews, I have contributed in the development of lead-free hybrid perovskites, such as CH3NH3Bi2I9, microstructural and photophysical properties of A-site cation aliovalent non-stoichiometric (CH3NH3)1-2x(H3NC2H4NH3)PbI3, and CH3NH3PbBr3-xClx. In collaboration with the University of Edinburgh, I have contributed to the development of perylene diimide based novel electron transporting layers for hybrid perovskite solar cells
The novel charge transporting layers developed is beneficial towards the halide perovskite based optoelectronic devices such as LEDs, photodetectors etc. The high spatial resolution mapping of composition heterogeneity and the spatial distribution of non-radiative recombination centres identified will directly contribute to the stability improvement and the methods to mitigate the non-radiative recombination losses in halide perovskites. The indoor light harvesting of the hybrid perovskites demonstrated will advance the application of the Internet of Things technology (IoT) inside the buildings. These indoor light harvesters will partially recycle the electrical energy used for lighting up the buildings and improve the energy security of the nation. By supporting to develop the IoT technology inside the buildings, the quality of life and well-being of the society will be enhanced by advancing the better health-care systems.
